The purpose of this study is to develop techniques for studying cental nervous system monoamine metabolism in man, and to carry out pharmacokinetic studies of psychoactive drugs. Studies of the kinetics and clearance of deuterated homovanillic acid (HVA) from the circulation contained in Rhesus monkeys and patients. Problems with the application of this technique to study CNS dopamine turnover in man were resolved by further study of the kinetics and volume of distribution of the metabolite, HVA, before and after probenecid administration. The mono-hydroxy metabolites of phencyclidine (PCP) were found to be psychoactive in primate studies, and may account for the mixed stimulant and depressant effects of the parent drug. The plasma half-lives of the mono-hydroxy metabolites are about two hours, and the hydroxy-cyclohexane metabolite partitions rapidly into brain, achieving levels three times greater than those in plasma.